For prevention of air pollution, wet limestone-gypsum desulfurization systems have been widely put into practical use as systems that remove sulfur oxide and the like contained in flue gases. This flue gas desulfurization system is shown in FIG. 5. A flue gas emitted from a combustion apparatus such as a boiler is introduced into an absorption tower (absorber) 1 through a gas inlet port 2, and as a result of contact with liquid drops of an absorbent liquid of limestone, a slurry including lime, or the like sprayed from spray nozzles 5 respectively provided on spray headers 3 installed in the absorber 1 and provided in multiple stages along the direction of a flue gas flow, SOx in the flue gas is absorbed along with acid gases such as hydrogen chloride (HCl), hydrogen fluoride (HF), and the like in the flue gas. Simultaneously, soot and dust caused by components contained in the fuel used by the combustion apparatus and other substances are absorbed and removed by the absorbent liquid.
Moreover, mist made to accompany the flue gas is removed by a mist eliminator 7 installed in a gas outlet port 4 of the absorber 1, and a clean gas passes through the gas outlet port 4, is reheated as necessary and discharged from a chimney. An SOx absorbent, for example, limestone, is fed as a limestone slurry 8 into a recirculation tank 9 of the absorber 1 by an unillustrated limestone slurry pump according to the amount of SOx absorption.
The absorbent liquid in the recirculation tank 9 is extracted by a plurality of absorbent liquid recirculation pipes 11 that connect with the recirculation tank 9, raised in pressure by absorber recirculation pumps 12 provided on the recirculation pipes 11, sent to the spray headers 3 connected to the respective recirculation pipes 11, respectively, and then sprayed from the spray nozzles 5. The flue gas that rises inside the absorber makes gas-liquid contact with the sprayed absorbent liquid drops, and SOx and the like in the flue gas are absorbed and removed.
The SOx in the flue gas reacts with calcium in the absorbent liquid to form calcium sulfite (including calcium bisulfite) as an intermediate product, falls in the recirculation tank 9, and is oxidized into gypsum by air 14 fed to the recirculation tank 9 while being raised in pressure by an unillustrated air blower or the like to be an end product (gypsum). By thus directly feeding air into the absorber 1, an absorption reaction of SOx in the flue gas and an oxidation reaction of the produced calcium sulfite are made to progress simultaneously, whereby a desulfurization reaction as a whole can be promoted.
Also, the air 14 to be fed to the recirculation tank 9 for an oxidation reaction of calcium sulfite is miniaturized by an oxidizing agitator 15 to thereby increase the utilization rate of oxidization air. Thereafter, the absorbent liquid slurry is extracted from the recirculation tank 9 while being raised in pressure by a gypsum slurry bleedpump 16 according to the amount of produced gypsum, sent to gypsum dewatering system 17, and collected as powder gypsum.
In recent years, there have been an increasing number of cases where a large capacity of flue gas from a large-capacity boiler or a plurality of boilers is treated by one absorber for improvement in reliability of a wet flue gas desulfurization system and from an economic perspective. Moreover, when the concentration of SOx in a flue gas is high or when highly efficient desulfurization of a large capacity of flue gas or a flue gas with a high SOx concentration is required, the necessary desulfurization performance is satisfied by increasing the amount of circulation spray of the absorber.
The amount of liquid circulated through the absorber is increased particularly in a plant where highly efficient desulfurization of a large capacity of flue gas with a high SOx concentration is required. On the other hand, since there are limits in the capacity of absorbent liquid to be sprayed by the spray nozzles 5 attached to the absorber spray header 3, the installing number of spray nozzles 5 to be attached to each absorber spray header 3, and liquid feeding amount performance of the absorber recirculation pump 12, respectively, there is a limit to the amount of absorbent liquid to be circulatively fed to the spray nozzles 5 of each spray header 3, so that multiple stages of spray headers 3 are installed.
When there are three stages of spray headers 3 as shown in FIG. 5, due to a restriction in the capacity of the absorber recirculation pump 12, it is necessary to install one or, possibly, a plurality of absorber recirculation pumps 12 per one stage of spray header 3. Since the amount of liquid circulation is increased in the above-described flue gas desulfurization system that is capable of treating a large capacity of flue gas and that highly efficiently desulfurizes a flue gas with a high SOx concentration, it is necessary to provide a larger number of stages of spray headers 3 and a larger number of absorber recirculation pumps 12 than those conventionally provided.
Contrivances of an absorber to meet such a demand have been disclosed in Specification of U.S. Pat. No. 5,620,144 and Japanese Published Unexamined Patent Application No. 2003-175314.